The invention relates to a flux composition used in the electroslag refining process. In particular, the invention relates to a flux for such use which substantially precludes introduction of hydrogen into the resulting steel and at the same time maintains good desulfurization capacity.
The electroslag remelting process basically comprises the fusing, from a consumable electrode, of metal, which is refined in and under a blanket of molten flux and solidified in a surrounding mold. Generally this is accomplished by suspending a consumable electrode within the mold with its lower end received in molten flux which was previously introduced, the flux acting as a conductor for electric current passing between the consumable electrode and a base plate or stool at the bottom of the mold. The current passing through the electrode and molten flux causes a rise in temperature of the electrode, and droplets of metal melting off the electrode's lower end fall through the flux to the bottom of the mold, where a pool of molten metal forms and solidification of the metal takes place upwardly from the bottom of the pool. The molten metal undergoes a refining action as it passes through the molten flux and, as the electrode is progressively consumed, a refined metal ingot is formed which builds up from the bottom of the mold. The molten flux floats on the pool of refined metal and is maintained in contact with the lower end of the consumable electrode. The refining process continues until the electrode is substantially or entirely consumed.
For the electroslag refining process, the ideal flux composition should have an appropriate melting point, low vapor pressure, low viscosity, low starting moisture content, high electrical resistivity, capacity to produce good surface quality, capacity to maximize desulfurization, and capacity to prevent excessive oxidation of easily oxidizable alloying elements. Further, the flux should not transport hydrogen from the atmosphere to the liquid metal pool and should not produce unstable operating conditions in the electroslag refining process.
In early work on the flux chemistry, a flux composition which had as principal components, CaF.sub.2, Al.sub.2 O.sub.3 and CaO was employed. Good quality steel ingots were produced by using such chemistry. However, a problem persisted with a potential for the introduction of hydrogen into the remelted ingot. This could be caused by high starting moisture content of the flux and/or the absorption of moisture from surrounding air by the flux, disassociation of moisture into hydrogen and oxygen in the flux, and absorption of hydrogen by the molten steel passing through the flux and before it solidifies under the flux. To meet this problem, a flux having a chemistry of approximately 70% CaF.sub.2 and 30% Al.sub.2 O.sub.3 was utilized. This flux substantially reduced but did not eliminate the hydrogen problem. Secondly, this flux has a reduced desulfurization capacity in the electroslag remelting process as compared with a flux composition, the principal components of which are CaF.sub.2, Al.sub.2 O.sub.3, CaO. Furthermore it reduces the electrical efficiency, deepens the liquid metal pool and is corrosive to refractories employed in melting the slag prior to introducing same to the mold, all of which are undesirable to the process.